This invention pertains to a digitizing tablet integrally formed with a flat panel or other type of rear projection display, and more specifically with a fast-update, durable digitizing tablet having excellent spatial linearity and stability, and which integrates well with a projection display.
Transparent digitizing tablets are often intended for use with some sort of display system in order to provide the user with the appearance of writing on top of the displayed image. This may be done, for example, to facilitate outline tracing of some structure in a projected photographic image, to allow an input mode which emulates natural writing. In all of these applications, correspondence between the coordinate system of the transparent tablet and that of the display system is a desired, if not crucial, characteristic.
Prior art transparent digitizing tablets, such as the SPD series of devices manufactured by Scriptel Corporation of Columbus, Ohio, utilize magnetic, electrostatic, or acoustic techniques to specify the location of the stylus. Use of such a prior art transparent digitizing tablet as a combination display/digitizer, such as the FIOS-6440 device manufactured by Photron Ltd. of Tokyo, Japan, or the use of a transparent digitizing tablet such as the TNN-01 from Sony Corporation in conjunction with a CRT, can result in poor correspondence between the detected location of the stylus and the desired portion of the displayed image.
FIG. 1 shows a cross section of a typical prior art transparent digitizer tablet system including a digitizing tablet 11 located adjacent to liquid crystal display 12. Prior art system 10 includes a surface plate 13 (which may help to contain liquid crystal display 12) located above liquid crystal display 12 and upon which stylus 14 is placed, thus leading to problems with parallax, since the image is formed on liquid crystal display 12 located below the surface of surface plate 13. Regardless of the technique used to specify the location of the cursor with respect to the displayed image, the tablet must be aligned with respect to the image produced by the display, and may not, even when optimally aligned, provide coordinates which register well with those of the displayed image due to distortions in either or both the tablet or the image produced by the display. The additional difficulties due to parallax are described, for example, in "Handwriting Recognition on Transparent Tablet over Flat Display", Tappert et al., SID 86 Digest, pages 308-312. Additionally, such environmental effects as stray electric or magnetic fields and triboelectric charging can cause distortions and/or disruptions in the digitized stylus position data. Certain prior art devices, such as the "Summasketch" device available from Summa Graphics, the "Screenplay" device from Seiko, and the HDG 1111C device from Hitachi are electromagnetic tablets which require a sensing element in the stylus whose equivalent location does not correspond well to the physical location of the stylus tip, resulting in distortions in the reported coordinates which may vary with the position of the stylus or the angle at which it is held.
Thus, there remains the problem with prior art transparent digitizer tablets relating to correspondence between the intended location of the stylus and the corresponding point of the displayed image.